Eddy current testing detects changes in eddy current induced in an object under test and is sensitive to material properties of the object through their effect on resistivity and magnetic permeability. The eddy current is indirectly measured by a probe coil located near the surface of the object which monitors the magnetic flux created by the eddy current. However, when an eddy current probe is used for ferromagnetic tube inspection, the magnetic permeability of the ferromagnetic material affects the probe coils inductance as well as depth of eddy current penetration into the material. The magnetic permeability strongly depends on factors such as:
thermal processing history; PA1 mechanical processing history; PA1 chemical composition; PA1 internal stresses; and PA1 temperature (if close to Curie temperature).
The large variations in permeability make conventional eddy current testing for defects in magnetic materials very difficult. Thus, it is not that the eddy current probe is insensitive to a ferromagnetic material, but that is produces signals from defects as well as from permeability variation of the material. It is very difficult to analyze and separate signals by defects from those by permeability variation (permeability noise). One way of suppressing the permeability noise is to bring the magnetic material to a condition where .mu..sub.r =1.0. Relative incremental or recoil permeability, .mu..sub.r, is defined as .mu..sub.r =.DELTA.B/.DELTA.H where .DELTA.B is the change in flux density which accompanies a change in magnetizing force, .DELTA.H created for example by an eddy current coils' alternating current.
A few slightly magnetic materials can be heated above their Curie temperature to make them nonmagnetic. Monel.TM. 400 heated to between 50.degree. and 70.degree. C. has been tested in this manner. Most materials, however, have too high a Curie temperature to be tested by this approach. The only other way to decrease .mu..sub.r to unity is by magnetic saturation.
It should also be noted that typically, the in-situ inspection of a tube in a steam generator and in other types of heat exchangers or the like, must be made from the bore of the tube, as support members such as support plates, tube sheets, headers and the like, prohibit the inspection being made from the exterior of the tube. In addition, these support members introduce various noises in eddy current signals as they modify the distribution of magnetic flux, thus affecting eddy current generation. Furthermore, quite often during the production process, these support members create certain anomalies in the tube, e.g. tube expansion under tubesheet, concentric defects, etc.
In U.S. Pat. No. 2,124,579, Jul. 26, 1938, Knerr et al describe a method of and an apparatus for testing metallic articles, in particular tubular articles. It teaches an eddy current probe located between a pair of electromagnets. In one of the embodiments, the probe and the electromagnets are made in such a way that they can be passed inside the tube under inspection.
In U.S. Pat. No. 2,992,390, Jul. 11, 1961, De Witte likewise teaches a technique of testing ferrous pipes for pipe thickness, pitting or corrosion, etc.It employs a magnetization means to magnetically saturate the pipe and a transmit-receive coil combination to measure changes in permeability which in turn correlates to pipes' physical characteristics being sought after.
U.S. Pat. No. 3,091,733, May 28, 1963, Fearon et al also describes an apparatus for detecting flaws in elongated magnetic structures. It uses a magnetizing means to magnetize the structure and subsequently a magnetic field measuring probe is passed over to measure the magnetic flux leakage that exists at the flaw.
In all these prior art techniques, the magnetic field is generated substantially coaxially with the axis of the tube under inspection. It is essential that the magnetic field is uniform around the circumference of the tube to ensure uniform sensitivity thereabout. The uniform magnetization allows the eddy current probe to pick up a localized defect in the circumference no matter what the relative location of the defect and the probe is as long as the tube under inspection is reasonably small in diameter.
U.S. Pat. No. 3,940,689, Feb. 24, 1976, Johnson, Jr. uses a combination of eddy current measurement and flux leakage detection for inspecting borehole pipes. It employs a plurality of eddy current probes positioned in circumference of the tube to pick up localized flaws in the pipe which must be fairly large in diameter. The magnetization means is also located coaxially with the pipe to ensure a uniform concentric magnetic field. However, it is very doubtful that such a magnetization means described in the patent would be strong enough to magnetize sufficiently the bore hole pipe.
There have never been convenient eddy current probes which can distinguish localized defects from concentric anomalies in a relatively small-sized ferromagnetic tube.